How can I make characters more believable and lovable by readers?

Relatable = Lovable. Make the characters easy to relate to. Lovable doesn't necessarily mean a "good guy." Nearly all Star-Wars nerds 'love' Darth Vader. Think about your target audience (who will be reading the book or what type of people the book is meant for) and make the characters have similar traits to that genre of people... only more extreme. For instance, if a character is shy, make him/her so shy that it brings out a level of innocence in the character. That way it will provoke a sense of emotion in the reader, making the character 'lovable.'